Under normal operation, an integrated circuit, for example a microprocessor, generates heat that must be removed to maintain the device temperature below a critical threshold and thereby maintain reliable operation. The threshold temperature derives from many short and long term reliability failure modes and may be specified by a circuit designer as part of a normal design cycle. The evolution of integrated circuit designs has resulted in higher operating frequency, increased numbers of transistors, and physically smaller devices. This continuing trend generates ever increasing area densities of integrated circuits and electrical connections. To date, this trend has resulted in both increasing power and increasing heat flux devices. Further, the trend may be expected to continue into the foreseeable future.
The problem of maintaining device temperature below a critical threshold value may be addressed at various levels of packaging. For example, a heat sink may be a common board level component, a fan a common system level component, and a thermally conductive packaging material a common device level component. A design team may choose various combinations of device, board, and system level components when faced with a particular thermal challenge.
Consider a device level component. Various materials in an electronic package typically each have a unique bulk linear coefficient of thermal expansion. As a result, under normal operation, temperature variations within a package may cause the various materials to undergo different levels of thermal expansion or contraction and may thus result in mechanical stresses within the various materials. Thus, a component used to address a thermal challenge may force further design consideration.
For example, a copper heat spreader may be thermally coupled to a backside of a die using a solder. Copper has a bulk linear coefficient of thermal expansion (“CTE”) of approximately 16.5 ppm/° C. in contrast to silicon, which has a bulk linear CTE of approximately 2.6 ppm/° C. Thus, a unit volume of copper may expand considerably more than a unit volume of silicon. At room temperature, mechanical stresses resulting from solder attachment of a heat spreader to a die may thus depend on the solder's phase change temperature, with lower temperature solders resulting in lower mechanical stresses compared to higher temperature solders. See FIG. 1.
Further, an integrated circuit package may undergo several elevated temperature processes during manufacture. For example, a package may undergo a soldering process to attach a die to a substrate and the substrate may undergo a solder ball attach process. The assembled package may then undergo a solder process to attach the package to a motherboard. Often, each successive high temperature process occurs at a temperature lower than the previous one to avoid damage to an earlier completed portion of the package.